1. Field of the Invention
The invention relates in general to a measuring technique, and more particularly, to a circuit for detecting and measuring capacitance.
2. Description of the Related Art
Operating interfaces of recent electronic products have become more and more user-friendly and intuitive as technology advances. For example, via a touch screen, a user can directly operate programs as well as input messages/texts/patterns using fingers or a touch control pen, which is often easier than operating such programs or entering input via traditional input devices, such as a keyboard or buttons. In practice, a touch screen usually comprises a touch sensing panel and a display disposed at the back of the touch sensing panel. According to a position of a touch on the touch sensing panel and a currently displayed image on the display, an electronic device determines an intention of the touch to execute corresponding operations.
For a capacitive touch device, a touch affects an electric field of a touch point triggering a corresponding capacitance change. Thus, the sensitivity and accuracy of detecting and measuring the capacitance change are critical. Referring to FIG. 1A, a block 100 is a conventional capacitance detecting circuit. The capacitance detecting circuit comprises an operational amplifier 12, a pad 14, a feedback capacitor Cfb, two first switches SW1, and a second switch SW2. The pad 14 connects an external capacitor Cx to be measured outside the detecting circuit 100 to the operational amplifier 12. The capacitor Cx to be measured is located in a sensing panel, and reflects a capacitance change according to a user's touch.
In a first stage of the detection process, both of the first switches SW1 are turned on, and the second switch SW2 is turned off. The circuit in FIG. 1A is equivalent to a circuit shown in FIG. 1B. Under such conditions, the voltages at an output terminal Vout as well as positive and negative input terminals of the operational amplifier 12 are all equal to a reference voltage VL. Thus, the feedback capacitor Cfb is discharged to store no charge, whereas the capacitor Cx to be measured is charged to store charge Cx*VL. In a second stage of the detection process, both of the first switches SW1 are turned off, and the second switch SW is turned on. In this latter configuration, the circuit in FIG. 1A is equivalent to a circuit shown in FIG. 10. Under such conditions, the voltages at the positive and negative input terminals of the operational amplifier 12 both become a reference voltage VH. After redistributing the charge, the voltage at the output terminal is represented as:
                    Vout        =                  VH          +                                    (                              VH                -                VL                            )                        *                          Cx              Cfb                                                          (        1        )            
With known values of the reference voltages VL and VH and the feedback capacitor Cfb, a value of capacitor Cx to be measured can be deduced according to the output voltage Vout. By further dismantling the capacitor Cx to be measured into a background capacitance Cbg existent prior to the user influence and a capacitance change Csig generated by the users touch, Equation (1) can be rewritten as:
                    Vout        =                  VH          +                                    (                              VH                -                VL                            )                        *                          Cbg              Cfb                                +                                    (                              VH                -                VL                            )                        *                          Csig              Cfb                                                          (        2        )            
The background capacitance Cbg is substantially a fixed value, meaning that the main target to be measured is the capacitance change Csig. The feedback capacitance cannot be too small so as to prevent the output voltage Vout from saturation. However, it is observed from the last item in Equation (2) that the resolution of measuring the capacitance change Csig worsens as the feedback capacitance Cfb becomes larger.